This is an interesting point, and they do say:
What should I do if I accidentally sell/delete my digital items from the Explorer’s Pack or Imperial Edition?
Items granted in this manner are protected. While players can delete them, they will have to take extra steps ensuring their intentions. If they do accidentally sell or delete their digital items, players should contact our dedicated http://help.elderscrollsonline.com/ team.
Does this relate to selling to an npc or will selling to a player be possible? If you can sell them to a player it makes sense that they are limited to one per account and once sold that's it, you've lost your pet. Otherwise you could create and sell an infinite number by making new characters.